Color image recording technology has been investigated using ink jet recording process, electrophotography, silver halide light sensitive material, thermal transfer recording process etc. for obtaining a color hard copy. Among these thermal transfer image forming method has an advantage of easy operation or maintenance, small size, low cost and so on.
For forming color image using the thermal transfer image forming material, each three color ink layer of yellow, magenta and cyan colors. In obtaining a black image by using the yellow, magenta and cyan color, they do not give a flat absorption for all visible light range and therefore they give low light absorption area between each of yellow, magenta and cyan color because the conventionally used color of yellow, magenta and cyan color have high chromaticness.
So, it is not easy to reproduce a pure black by mixing such three colors and it has been known to utilize the fourth black ink layer in addition to yellow, magenta and cyan ink layers.
As for two dyes having different colors is disclosed in Japanese Patent Open to Public Inspection (referred as Japanese Patent OPI) Nos. 5-185752 and 5-201147. The conventional black ink layer is so designed as to absorb at whole visible light area by making use of plural dyes having different absorption maximum. However, difference of color tone between the high density portion and low density portion is found since the thermal mobility caused by applied energy of each dyes is different in density and gradation change according to change of applied energy. Further difference of color tone between the high density portion and low density portion is found and it has been difficult to obtain a stable neutral gray because the durability against light irradiation of each dye is different.
It is necessary to incorporate dyes with high density to obtain a flat black in case that plural inks are used in a black image forming material. In these occasions, there are other problems that deterioration of recording material during storage such that the dye deposit during the storage or transportation of the recording materials, blocking occurred.
When a single dye has flat absorption for whole visible light range, the problem will be dissolved. However, it is not practically used because it has such disadvantages that the flat absorption is not obtain, high sensitivity is not obtain because of low solubility and low thermal mobility, desirable storability of the dye image is not obtained.
While dyes are important in the thermal transfer recording method, conventional dyes have such disadvantage of low image stability, low fixing ability or low storability against light.
A method for forming an ink layer by mixing two dyes having different colors is disclosed in Japanese Patent OPI Nos. 5-185752 and 5-201147. In this method it has such disadvantage of low image stability, low fixing ability or low storability against light because usual thermal diffusible dyes are used.
Improvement of the dyes in easy synthesis and good solubility in a solvent is desired for usage in ink.